A known changing device of this kind is described in EP-A 0 125 529. It is important inn such a changing device that the changing head, when in the changing position, is always spaced the same distance from the axis of the spindle, since only then is it ensured that the axis of the tool holder is in alignment with the axis of the spindle during tool changing. Since therefor the end position of the changing head with respect to the axis of the spindle in the known changing device is always the same, tool holders can be interchanged only if their cylindrical parts are of the same diameter. Usually, the cylindrical part is a portion of the shank of the tool holder which is supported by means of the said shank against the end surface of the spindle. Although it has been attempted to make the diameter of this shank as large as possible with a view to providing stability, it may be preferable, in the case of tools which are subjected to relatively small loads, such as reaming bits, small diameter drill bits, screw-thread cutting tools, etc., to make the shank smaller in diameter so that the tool holder is lighter and hence, because of its smaller size, is quicker to change and takes up less space in a magazine. A shank of smaller diameter inevitably has, however, the result that the cylindrical part of the tool holder in which the changing head is engageable will also be of smaller diameter. The known changing device is unsuitable for the interchangeable engagement of tool holders having cylindrical parts of different diameters.
The basic object of the invention is accordingly to provide a toolchanging device for a machine tool of the kind initially referred to which enables tool holders having cylindrical parts of different diameters to be interchanged, while the end position of the changing head with respect to the axis of the spindle remains constant.
This object is achieved according to the invention in that the changing head has, at the end thereof which faces towards the tool holder concerned at any time, a substantially segmented part and a first centring recess in the latter formed by a first part-cylindrical (arcuate) centring surface the radius of which corresponds approximately to the radius of the cylindrical part of a first tool holder, that portions of a second part-cylindrical (arcuate) centring surface are provided, one on each side of the segmental part, on a part of the changing head which is widened out axially of the latter, the said second centring surface being concentric with the first centring surface and having a radius corresponding approximately to the relatively large radius of a cylindrical part of a second tool holder which has, in the vicinity of its radial screw-threaded bore, a recess for the segmental part.
With this new tool-changing device, tool holders, which have cylindrical parts of different diameters, can be interchangeably received and replaced. If the tool holder has a cylindrical part of relatively small diameter, then the first centring surface engages this cylindrical part. If, however, another tool holder has a cylindrical part of relatively large diameter, then the changing head engages the cylindrical part by means of its second, larger radius centring surface. The recess in the cylindrical part is provided so that the segmental part, which in this case is superfluous and inoperative, can enter the said recess. The interchangeable tool holder is located in this case only with its cylindrical part in contact with the second centring surface parts of the changing head. Since these second centring surface parts are concentric with the first centring surface, the centring surfaces have a common centre of curvature or a common axis of curvature which, so long as the end position of the changing head with respect to the axis of the spindle remains unchanged, coincides with the latter.